1. Field of the Invention
This invention relates to a method and apparatus for determining well logs from downhole differential or gradient measurements. More specifically, the invention relates to a method and apparatus for determining the spontaneous potential voltage existing in earth formations traversed by a well bore.
2. Description of the Prior Art
The spontaneous electrical potential, or self-potential (SP), is the naturally produced galvanic potential phenomenon produced at the interface of the drilling mud in the well bore and the well formations in conjunction with the interface between dissimilar but adjacent earth formations. Measurement of the self-potential as a diagnostic approach in determining characteristics of earth formations has been in extensive public use since about 1935. Knowledge and understanding of the self-potential has slowly evolved from an original mere recognition of its existence to its present day quantitative use.
There are many factors which have complicated self-potential interpretation and restrained its use as an absolutely reliable guide to formation bed characteristics and boundaries. Such factors may be the well bore diameter, drilling mud filtrate invasion, properties of the formation fluids, and characteristics of the subsurface data. Salts, carbonates, and fresh water greatly modify and reduce the quantitative use of the self-potential measurement. Shaley sand formations create conditions hampering self-potential interpretation. An even greater problem is electrical "noise" which is the predominate source of low quality and erroneous self-potential measurements and resulting low quality SP log curves.
A principle source of such electrical noise is related to variations in the ground potential at the earth's surface to which the well bore spontaneous potentials are usually referenced. The causes of such noise include disruptive electrical currents adjacent to the earth's surface near the well bore, magnetization of the well logging equipment at the earth's surface, inadequate ground connection with reference to the potential measurement electrode, and related factors.
Often the exact influence of some or all of these factors affecting spontaneous potential response cannot be known, predicted, or eliminated. Consequently other tools, where available, are used in conjunction with SP logging equipment to determine the desired formation information. Focused induction resistivity determining devices are commonly used for this purpose.
Despite the limitations of the self-potential log, there are a number of occasions and areas where it satisfactorily reflects permeable bed boundaries and, when auxiliary logs are not available to refine the self-potential measurement, the self-potential is used as the primary indicator of bed boundaries. In these cases, the method of recording, operating on, and displaying SP data as provided by the present invention can provide both supplemental and enhanced variations on the conventional SP curve.
Various methods for measuring spontaneous potential have previously been disclosed. In U.S. Pat. No. 2,992,389 one or more toroidal coils of saturable magnetic cores are employed to detect induced current flow. In U.S. Pat. No. 3,268,801 two relatively close electrodes are used to drive galvanometers with attached mirrors and optical recording device to measure the electrical potential drop across the electrodes during drilling. In U.S. Pat. Nos. 3,638,105, and 3,638,106 a three electrode system producing an SP log is disclosed wherein one downhole exploring electrode is referenced to a downhole local ground electrode with low frequency filtering to produce a high frequency contribution to SP and is also referenced to a surface electrode with high frequency filtering to produce a low frequency contribution to SP. Similarly, U.S. Pat. Nos. 3,691,456 and 3,715,653 describe methods for converting such three electrode SP measurements from a function of spacial coordinates to a function of time.
A rather comprehensive paper by E. A. Koerperich, entitled "A DOUBLE-ELECTRODE METHOD FOR SPONTANEOUS POTENTIAL LOGGING", has been published in the November 1979 issue of Journal of Petroleum Technology. The method presented in Mr. Koerperich's paper is acknowledged as being directly derived from a single conventional SP curve measurement or from two conventional SP curves simultaneously recorded at different depths in the well bore with the exploring electrode or electrodes being referenced to a surface ground electrode. A so-called "Delta SP curve" is then constructed as the difference between the two conventional SP curves. In addition to the Delta SP curve being useful in conjunction with the conventional SP curve, certain advantages over the conventional SP curve are suggested including the improved determination of bed boundary locations and bed thicknesses, particularly when surface generated noise signals exist on the SP curve. However, the article concludes, "The system would not be expected to eliminate noise that could be introduced on one electrode signal independent of the other signal--as in the case of noise introduced on the collector where individual signals are picked off the conductors and transferred to the panel."
Further insight into the noise problem associated with a direct gradient or differential SP measurement appears in the previously mentioned U.S. Pat. No. 3,638,105 where it is stated, "To produce a noiseless SP log, it has been previously suggested to measure the differential SP or SP gradient; i.e., the differenence in potential between two relatively closely spaced downhole electrodes, and integrate this difference in potential to arrive at the trace SP value. However, any DC-type noise caused by polarization, or amplifier zero error, will be converted into a very large error in a relatively short time by the integrator in such a system."